1. Field of the Invention
This invention relates to cleaning metal surfaces, both painted and unpainted, more particularly when the metal surfaces are adjacent to elements of plastic, especially polycarbonate plastic, which is susceptible to stress cracking when repeatedly contacted with many conventional metal cleaning compositions. This invention also relates to cleaning compositions that are useful in such cleaning processes, are not overly acidic or alkaline, and are not susceptible to developing undesirable odors from micro-organisms that readily come into contact with the compositions during normal storage or use.
2. Statement of Related Art
A very wide variety of cleaning compositions are known in the art. Few if any of these compositions, however, are known to be capable of fulfilling all the desiderata noted above, particularly for railroad cars that are powered by electricity. Such cars, like other railroad cars, come into contact with a wide variety of atmospheric pollutants and air-borne soils that often lead to rapid deterioration in the aesthetic appearance of the exteriors of the cars. Ordinary rail cars that do not supply their own motive power can be readily cleaned by a variety of cleaners, usually most readily by fairly highly alkaline cleaners. However, rail cars that are directly powered by electricity, a type especially frequently used in mass transit operations, generally have housings of polycarbonate plastic on their exterior surfaces to protect electrical contacts that supply power to move the cars from a "third rail", overhead power line, or the like. This plastic readily develops stress cracks when contacted repeatedly by many alkaline solutions, including some of the most generally effective aqueous metal cleaning compositions. Such large vehicles can most conveniently be cleaned by sprayers, but it is a practical impossibility in many cases to protect any plastic parts of the exterior surface from contact with a sprayed cleaner composition, and frequent replacements of the plastic insulating housings are economically unacceptable.